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Many migratory species have shifted their geographic distribution in response to climate change, yet the underlying mechanisms are poorly understood, particularly for mammals. We hypothesized that generational shifts are underlying the observed colonization of hibernation sites further north in a migratory bat, the common noctule ( Nyctalus noctula) . To evaluate our hypothesis, we collected long-term data on the migratory status and demography of common noctules in a recently colonized hibernation area. Based on isotopic data of 413 individuals, we observed a significant decline in the proportion of long-distance migrants from 2004 to 2015 for both sexes and across all age groups. Demographic data collected between 2007 and 2016 from 3394 individuals demonstrated that subadult males were more abundant during the early colonization stage, followed by a gradual shift to a more balanced age and sex composition. Our results suggest that the colonization of hibernacula at higher latitudes is promoted by generational shifts, involving mostly first-year males. Generational shifts seem to be a likely mechanism for distribution changes in other bats and potentially also in other mammals.

Female bats of temperate zones often communally rear their young, which creates ideal conditions for naive juveniles to find or learn about resources via informed adults. However, studying social information transfer in elusive and small-bodied animals in the wild is difficult with traditional tracking techniques. We used a novel ‘next-generation’ proximity sensor system (BATS) to investigate if and how juvenile bats use social information in acquiring access to two crucial resources: suitable roosts and food patches. By tracking juvenile–adult associations during roost switching and foraging, we found evidence for mother-to-offspring information transfer while switching roosts but not during foraging. Spatial and temporal patterns of encounters suggested that mothers guided juveniles between the juvenile and the target roost. This roost-switching behaviour provides evidence for maternal guidance in bats, a form of maternal care that has long been assumed, but never documented. We did not find evidence that mothers guide the offspring to foraging sites. Foraging bats reported brief infrequent meetings with other tagged bats that were best explained by local enhancement. Our study illustrates how this recent advance in automated biologging provides researchers with new insights into longstanding questions in behavioural biology.

Migratory decisions in birds are closely tied to environmental cues and fat stores, but it remains unknown if the same variables trigger bat migration. To learn more about the rare phenomenon of bat migration, we studied departure decisions of female common noctules ( Nyctalus noctula ) in southern Germany. We did not find the fattening period that modulates departure decisions in birds. Female noctules departed after a regular evening foraging session, uniformly heading northeast. As the day of year increased, migratory decisions were based on the interactions among wind speed, wind direction and air pressure. As the migration season progressed, bats were likely to migrate on nights with higher air pressure and faster tail winds in the direction of travel, and also show high probability of migration on low-pressure nights with slow head winds. Common noctules thus monitor complex environmental conditions to find the optimal migration night.